Microwave is an electromagnetic radiation which has wavelengths shorter than radio waves and longer than infrared radiation. The frequencies allocated for industrial drying are 896 MHz and 2450 MHz. Most molecules, although electrically neutral, have an asymmetrical distribution of electrons and may be electrically positive at one end and negative at the other. Water molecule also has a dipolar property because the electrons are more concentrated on the oxygen atom within the molecule. In an electric field, the molecules align themselves in a specific direction and when the field is removed, it returns again into its random orientation. A microwave radiation creates a rapidly pulsating electrical field, which rapidly changes the molecular orientation back and forth. The resultant molecular vibration produces heat. This is the principle for microwave heating and drying. In microwave dryers, microvave radiation is generated by magnetron type vacuum oscillators.

In continuous microwave dryers, the Stevia leaves are passed through a tunnel on a conveyor belt. The conveyor belt and the construction material of the inside of the tunnel is made of microwave resistant material. In the tunnel, the leaves are exposed to a series of microwave sources and the water molecules in the fresh Stevia leaves are heated up to the boiling point of water. The water in the leaves is thus evaporated at high temperature and the water vapour is removed from the tunnel by air blowers. The microwave energy, belt speed and the leaf feed speed is precisely controlled by microprocessors/computers so that the leaves are dried properly and are not exposed to unnecessarily high temperature. The industrial microwave dryers can give a throughput of few tons per hour.